Sensor with improved shelf life

ABSTRACT

The present invention provides a metal electrode stabilised by a coating, the coating comprising a sulfur containing moiety in its molecular structure. The coating may also include a hydrophilic group and a spacer between the sulfur containing moiety and the hydrophilic group. Preferably, the sulfur containing moiety is selected from the group comprising thiol, disulfide and SO x , and the hydrophilic group is selected from the group comprising hydroxyl, amine, carboxyl, carbonyl, oligo (ethylene oxide) chain, and zwitterionic species. Compounds useful in the present invention include 2-mercaptoethanol, 2-mercaptoethylamine, 3-mercaptopropionic acid, thiophene, 4-carboxythiophene, cysteine, homocysteine, and cystine.

RELATED APPLICATIONS

This application is a continuation of PCT Application No. PCT/AU99/00166, filed on 16 Mar. 1999, which claimed priority from Australian Application No. PP 2503, filed on 20 Mar. 1998.

TECHNICAL FIELD

The invention relates to apparatus comprising one or more metal electrodes such as electrochemical cells, sensor elements and the like, and more particularly to extending the shelf life of such apparatus.

BACKGROUND ART

Metal electrodes have proved useful in sensor elements for sensing a diverse range of biologically important molecules eg glucose, and for determining physical properties such as pH. A range of possible configurations and applications involving metal electrodes are discussed in our co-pending applications PCT/AU96/00210, PCT/AU96/00365 and PCT/AU96/00723.

A desirable attribute of all sensor elements is that they have a long shelf life—that is, the sensing characteristic of the sensor element does not change significantly between manufacture and use (ie on storage).

In an electrochemical sensor element the stability of the electrode is critical to the stability of the sensor as a whole. Typically, when left to stand for long periods of time, electrodes become prone to instability in subsequent use thus limiting the useful shelf life. It is thought that such instability is caused by absorption or reaction of the metallic surface with atmospheric contaminants. It has also been observed that filling time of sensors deteriorates on prolonged storage.

It is an object of the present invention to overcome or ameliorate at least some of the above disadvantages in the prior art.

Surprisingly, the present applicant has found that by coating a metal electrode with a monolayer or multilayer of selected materials, electrode behavior can be significantly stabilised in comparison with uncoated metal electrodes without loss of the desirable sensing characteristics of the electrodes.

DESCRIPTION OF THE INVENTION

According to a first aspect, the invention consists in a metal electrode stabilised by a coating, said coating comprising a sulfur containing moiety in its molecular structure, said coating increasing the temporal stability of the electrode relative to a corresponding uncoated metal electrode without modifying other electrochemical properties of said metal electrode.

“Comprising” as herein used is used in an inclusive sense, that is to say in the sense of but not limited to “including” or “containing”. The term is not intended in an exclusive sense (“consisting” of or “composed of”).

Preferably, the sulphur-containing moiety is selected from the group comprising thiol, disulphide and SO_(x). Most preferably the sulphur-containing moiety is a disulphide. The sulphur-containing moiety may also be incorporated in a cyclic structure.

According to a second aspect, the invention consists in a metal electrode stabilised by a coating according to the first aspect, further comprising a hydrophilic group in its molecular structure.

Preferably, the hydrophilic group is selected from the group comprising hydroxyl, amine, carboxyl, carbonyl, oligo (ethylene oxide) chain, and zwitterionic species. Most preferably, the hydrophilic group is a zwitterionic species. The most preferred zwitterionic species comprises an amine and a carboxyl group in proximity.

According to a third aspect, the invention consists in a metal electrode stabilised by a coating according to the second aspect, further comprising a spacer between the sulphur-containing moiety and the hydrophilic group.

Preferably, in the third aspect, the spacer consists of an alkyl group or an aromatic group. It is preferable that methylene or ethylene groups be included in the spacer element.

According to a fourth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating, comprising the step of contacting a metal electrode with a substance comprising a sulphur-containing moiety in its molecular structure.

According to a fifth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating, comprising the steps of contacting a metal electrode pith a substance comprising a sulphur-containing moiety and a hydrophilic group in its molecular structure.

According to a sixth aspect, the invention consists in a method of preparing a metal electrode stabilised by a coating comprising the steps of contacting a metal electrode with a substance comprising a sulphur-containing moiety, a hydrophilic group and a spacer between the sulphur-containing moiety and the hydrophilic group in its molecular structure.

The preferred substances for use in the methods described in the fourth, fifth and sixth aspects are identical to those substances described in respect of the first, second and third aspects.

The invention also consists in a method of sensing an analyte, comprising the step of substituting the electrode in a known sensor device with a metal electrode stabilised by a coating according to the present invention, and sensing an analyte.

BEST MODE FOR CARRYING OUT THE INVENTION

Various embodiments of the invention will now be described by way of example only.

It is known in the prior art that thiols form coatings on metals. Thiols have also been used to tether species such as antibodies onto metal surfaces, for instance those of gold particles, for the purposes of immobilisation etc. One would expect that such coatings would also bind contaminants to the surface.

As much electrode chemistry involves interaction at the electrode surface, it is thus surprising that coatings used to bind molecules to the metal surface can be useful in preventing contamination of the electrode surface. It is also surprising that notwithstanding the application of the coating an electrode retains desirable electrochemical properties. The procedure for preparing the metal electrode stabilised by a coating involves contacting a metal electrode with selected sulphur-containing compounds, such as thiols, disulphides and compounds of the formula SO_(x) among others being suitable in the context of the present invention. The coatings also desirably contain a hydrophilic group which includes such species as hydroxyl, amino, carboxyl, carbonyl, oligo (ethylene oxide) chains and zwitterionic species. The latter two compounds indicate that compounds having one or more hydrophilic groups are also suitable groups for use in the present invention

Between the sulphur group, which acts to tether the molecule onto the metal surface, and the hydrophilic group, which presents a hydrophilic surface, spacers may be employed.

Compounds useful in the present invention include, but are not limited to 2-mercaptoethanol, 2-mercaptoethylamine, 3-mercaptopropionic acid, thiophene, 3-carboxythiophene, cysteine, homocysteine and cystine. Most preferably the molecule is cystine. In any of the above aspects, the D or L isomers can be used or mixtures of D and L isomers can be used, where such isomers are possible.

The compound in accordance with the invention is then applied as a monolayer or multilayer onto the surface of the electrode. It is possible to apply the compound by simply exposing the electrode to the coating material, with the coating material in either the vapour phase or in solution. The substance can be applied by dipping, spraying, painting, printing etc. After application, it is possible to wash the surface of the contacted electrode.

In a further aspect of the current invention the layer of the sulfur containing compound can optionally be overcoated with a surfactant layer. The surfactant layer can be applied after the application of the sulfur containing layer or at the same time as the sulfur containing layer, for example the sulfur containing species and the surfactant can be placed in a coating bath into which the electrode material is immersed. Due to the higher affinity of the sulfur containing species for the electrode material it will bind to the electrode surface in preference to the surfactant, leaving the surfactant in a layer over the sulfur containing layer. An example of a suitable surfactant is Triton X-100.

EXAMPLES Example 1 Preparation

The electrode coatings were applied to gold or palladium electrodes by immersing the sheet of material from which the electrodes were made into a 1 mM aqueous solution of the coating compound adjusted to pH 12 by the addition of potassium hydroxide. The contact time between the electrode material and the coating bath was typically 30 seconds. After coating, the electrodes were washed by immersion in a bath of water. In some cases, the electrodes were immersed in a third bath containing 1,000 ppm of triton X-100 in water. Finally, the electrode material sheets were dried by blowing with air at room temperature.

Example 2 Storage

The data in Tables 1 and 2 below show the effect on the electrode stability of coating the electrodes with sulphur-containing compounds. The stability was assessed using an accelerated test. The glucose sensors using coated or uncoated electrodes were stored either at 4° C. in the refrigerator (“fridge”) or at 56° C. in an oven for two weeks. The sensors stored at 4° C. do not change significantly from their performance when freshly prepared and tested. Those stored in the oven are subject to accelerated ageing, which simulates longer ageing times at room temperature.

Example 3 Testing

After to weeks the sensors were tested with whole blood samples with various glucose concentrations, from about 3 mM to 30 mM. The background ferrocyanide concentration was measured (the reading obtained when a sample contains no glucose) and the overall precision and fill speed of the sensors was assessed. The effect of the electrode coatings is shown in Table 1. The fill speeds in Table I were assessed qualitatively by eye. The fill speeds in Table 2 were assessed quantitatively by videoing the filling of the sensor with a blood sample using an on-screen timer and subsequently determining the number of seconds required for the blood to fill each sensor.

It can be seen from the first pair of results, for a non-coated electrode, that artificial ageing dramatically increased the % cv (corresponding to decreased precision).

In contrast, for the last two pairs of results, the % cv's for the treated electrodes after artificial ageing were comparable to the % cv's of untreated electrodes on fridge storage and significantly better than accelerated aged untreated electrodes.

A desirable side effect of the present invention also appears to be maintenance of good fill speed for sensors on ageing.

TABLE 1 TEST DATA BACKGROUND (mM MEAN FILL STORAGE COATING ferrocyanide) % cv SPEED Fridge None 1.01 3.8 OK Oven None 5.12 10.05 very slow Fridge Cysteine 1.3 4.5 OK Oven Cysteine 5.0 8.0 slow Fridge Cysteine/trit 1.98 3.1 fast Oven Cysteine/trit 2.17 5.4 OK Fridge Homocysteine/ 1.02 4.6 OK trit Oven Homocysteine/ 2.34 4.2 faster than trit Cysteine/trit Fridge Cystine/trit 0.63* 4.1 fast Oven Cystine/trit 1.24* 4.4 good *saline rather than blood used to assess the background Trit denotes an overcoating of Triton X-100.

TABLE 2 PRECISE FILL TIMES STORAGE COATING FILL TIME (secs) Fridge none 1.0 Oven none 5.3 Fridge Cystine 0.4 Oven Cystine 4.0 Fridge Cystine/trit 0.3 Oven Cystine/trit 1.4

A person skilled in the art will appreciate that the application process is very simple and facile and could be accomplished from the teaching hereof in many ways. 

The claims of the invention are as follows:
 1. A metal electrode stabilized by a coating, said coating comprising 3-carboxythiophene and adapted to increase a temporal stability of said electrode relative to a corresponding uncoated metal electrode, said coating further comprising an overcoating of a surfactant.
 2. A method of sensing an analyte including the step of providing a metal electrode stabilized by a coating according to claim 1, and sensing an analyte.
 3. A metal electrode according to claim 1, wherein said surfactant further comprises a hydrophilic group.
 4. A method of preparing a metal electrode stabilized by a coating, including the step of contacting a metal electrode with a substance comprising 3-carboxythiophene, said method further comprising the step of overcoating the substance with a surfactant.
 5. A method according to claim 4 wherein the surfactant and substance are applied simultaneously.
 6. A method according to claim 4 wherein the surfactant is applied to the electrode subsequent to the application of the substance.
 7. A method according to claim 4, wherein said coating is applied as a monolayer or multilayer on a surface of an electrode.
 8. A method according to claim 4, wherein said substance is applied by deposition from a vapor phase.
 9. A method according to claim 4, wherein said substance is applied by deposition from a liquid solution.
 10. A method according to claim 4, wherein said substance is applied by a method selected from the group consisting of dipping, spraying, painting, and printing.
 11. A method of sensing an analyte, said method comprising a step for preparing a metal electrode according to claim 4, and a step for sensing an analyte. 